1. Field of the Invention
The present invention relates to plugs used in oil and gas wells and in particular, though not exclusively, to an actuating mechanism which provides for controlled opening of a plug.
2. Description of Related Art
During the lifetime of an oil/gas production well, various servicing operations will be carried out to the well to ensure that the efficiency and integrity of the well is maximised. This would include; a full work over, surface well-head tree change, side tracking or close proximity drilling operations. To allow any of these operations to be done safely and to accommodate verification pressure tests from surface, it is necessary to install a plug (or plugs) into the production tubing to create a barrier to both test against and provide isolation from the production zones.
These plugs are typically installed/retrieved from the well bore by either wire line or coiled tubing methods. Wire line and coiled tubing operations however, can be time consuming and risky depending on the application, and are generally kept to a minimum where possible. When retrieving plugs it is necessary to equalise pressure above and below prior to unlocking and removal—this often involves an extra intervention run to initiate equalisation prior to retrieval.
One type of plug developed to remove the requirement for intervention is referred to as a pump open plug. This device is equalised by applying pressure to the tubing above the plug to a pre-determined value. This causes a specially rated shear pin to fail, actuating the device to communicate pressure between the tubing above and below the plug. Retrieval of the plug can then commence, or the plug left in situ and the well produces through the now open plug. This is a simple design which can be equalised remotely by pressure from the surface. It can also handle over balanced situations i.e. the pressure below the plug is always less than that above due to the hydrostatic weight of fluid above being greater than the zonal pressure below the plug.
However, this plug does have a number of disadvantages, namely that it does not allow for a full pressure test of the production tubing above the plug as the shear pin rating inherently has to be less than the production tubing's pressure rating. There is also a need to know what the expected pressure below the plug will be prior to opening as this is important when rating the shear pin. Additionally, the over balance conditions permanently load up the shear pin. Shear pins are inherently difficult to manufacture accurately and the shear pin used cannot be tested prior to installation. When the shear pin fails during opening operations the pressure can surge into the zonal formation causing formation damage within the well.
Pressure cycle plugs have also been developed. Such designs are those disclosed in GB 2,281,752 and EP 0,485,243. These are generally referred to as pressure cycle plugs. In such devices the pressure is equalised by applying, from surface, a predetermined number of pressure cycles (pressure up-bleed off). The actual value of pressure applied is less important than that of the pump open plug, it equivalently just needs to be more than the pressure below the plug. During each cycle applied, the equalisation mechanism with the device moves incrementally typically via a ratchet. On the last cycle the mechanism will finally move to a position that will allow communication to occur between the tubing above the plug to that below. Again retrieval of the plug can then commence, or the plug left in situ and the well produced with the now open plug. These plugs are advantageous in that the pressure can be equalised remotely from the surface. The value of the pressure applied is less critical than that needed for operating a pump open plug and the number of pressure cycles can be pre-set before the plug is installed, to allow enough scope to do all the pressure testing etc prior to opening. The plug will open during the bleed off phase of the pressure cycle and thus pressure surges to the formation are minimised. The tubing above the plug can be tested to the maximum pressure rating and then cycled open to a lower pressure.
While the pressure cycle plug has these advantages, it also has a number of disadvantages. A major disadvantage is that by virtue of the fact that a predetermined amount of cycles have to be undertaken before opening, this can be restrictive in well operations. Often during surface operations, pressures may be applied inadvertently to the tubing and it becomes confusing as to whether they constituted a cycle or not, therefore it becomes less clear how many cycles are left to open the plug. In order to operate the plug a knowledge of the pressure below the plug is required. Because the plug opens during bleed-off, it is not easy to tell if the plug was closed or open until the next cycle is applied. Therefore it is never clear if the plug is really closed without using up another cycle. Shock loading during installation of the plug can cause the internal mechanism to incrementally move, thus using up some cycles without knowledge by the operator. The internal mechanisms are not particularly suitable for use in over balance situation due to the hydrostatic weight of fluid above being greater than the zonal pressure below the plug.